Olefin polymers or copolyners such as polypropylene or polyethylene are excellent in mechanical properties and chemicals-resistance and are very useful in balance to an economic aspect so that they are employed in the field of various moldings. In the past, these olefin polymers or copolymers were produced by polymerizing or copolymerizing olefins by the aid of a so-called Ziegler-Natta catalyst which is a combination of a transition metal catalytic component comprising titanium trichloride and/or tetrachloride carried on a support such as magnesium chloride with an organoaluminum compound.
In recent years, on the other hand, a process for producing olefin polymers or copolymers by polymerizing or copolymerizing olefins by the aid of a new catalyst different from the conventional catalyst system has been carried out, the new catalyst being comprised of a metallocene and an aluminoxane. Olefin polymers or copolymers produced by way of this metallocene catalyst system are distinguished by their narrow molecular weight distribution and by the fact that in case of copolymers, the comonomer has homogeneously been copolymerized therein so that olefin polymers or copolymers which are more homogeneous than the conventional olefin copolymers or copolymers can be obtained. However, olefin polymers or copolymers obtained by way of these metallocene catalysts are lower in melt strength and may be limited in use according to the fields of application, as compared with olefin polymers or copolymers obtained by way of a conventional catalyst system.
As means for enhancing melt strength and elevating crystallizing temperature of polypropylene, there are disclosed a process wherein polypropylene is reacted with an organic peroxide and a crosslinking assistant in molten state (Japanese Laid-open Patent Appln. No. Sho. 59-93711 and Japanese Laid-open Patent Appln. No. Sho. 61-152754), and a process wherein a semi-crystalline polypropylene is reacted with a low decomposition temperature peroxide in the presence of oxygen to produce polypropylene having long chain branches and containing no gel (Japanese Laid-open Patent Appln. No. Hei. 2-298536).
As another means for enhancing melt strength and melt viscoelasticity, there are proposed a composition incorporated with polyethylene or polypropylene different in intrinsic viscosity or molecular weight or a process for producing such composition according to a multi-stage polymerization.
Such examples include, for example, a process wherein 100 parts by weight of ordinary polypropylene is incorporated with 2-30 parts by weight of ultra-high molecular weight polypropylene and a mixture is extruded at a temperature above the melting point but lower than 210.degree. C. (Japanese Patent Publn. No. Sho. 61-28694), an extruded sheet comprising two different polypropylene components of a limiting viscosity ratio of at least 2 obtained according to the multi-stage polymerization method (Japanese Patent Publn. No. Hei. 1-12770), a process for producing a polyethylene composition comprising 3 kinds of polyethylene different in viscosity average molecular weight containing 1-10% by eight of polyethylene of a high viscosity average molecular weight according to melt-kneading method or a multi-stage polymerization method (Japanese Patent Publn. No. Sho. 62-61057), a process for producing polyethylene wherein ultra-high molecular weight polyethylene having at least a limiting viscosity of 20 dl/g is polymerized by 0.05-1% by weight according to the multi-stage polymeion method by the aid of a highly active titanium-vanadium solid catalyst component (Japanese Patent Publn. No. 5-79683), and a process for producing polyethylene wherein ultra-high molecular weight polyethylene having a limiting viscosity of at least 15 dl/g is polymerized by 0.1-5% by weight according to the multi-stage polymerization method using a polymerization vessels of a specific configuration by the aid of a highly active tinum catalyst component subjected to a preliminary polymerization treatment with 1-butene or 4-methyl-1-pentene (Japanese Patent Publn. No. Hei. 7-8890).
Further known are a process for producing polypropylene of high melt strength by polymerization of propylene by the aid of a preliminary polymerization catalyst derived from a supported-type titaiium-containing solid catalyst component and an organoaluminum compound catalyst component subjected to a preliminary polymerization treatment with ethylene and a polyene (Japanese Laid-open Patent Appln. No. Hei. 5-222122) and a process for producing a linear low density polyethylene (LLDPE) of high melt strength by the aid of a preliminary polymerization catalyst containing polyethylene of a limiting viscosity of at least 20 dl/g obtained by a similar catalyst component subjected to preliminary polymerization with ethylene alone (Japanese Laid-open Patent AppIn. No. Hei. 4-55410).
As an attempt to enhance melt tension in case of using the metallocene catalyst system, there are proposed a process wherein a catalyst comprising a silica support containing 1.0% by weight of water, a metallocene, a methylalumiinoxane and triisobutylalunanum is used (Japanese Laid-open Patent Appln. No. Hei. 5-140224), a process wherein two kinds of metallocene compounds are used as catalyst component (Japanese Laid-open Patent Appln. Nos. Hei. 5-255436, 5-255437, and 6-206939) and a process wherein montrnorillonite is used in the metallocene catalyst system (Japanese Laid-open Patent Appln. No. Hei. 7-188336).
In these various compositions or a process for producing thereof, however, enhancement of melt strength to a certain degree may be recognized under the measurement condition of polyolefins at 190.degree. C., nevertheless problems still remain in obtaining enhancement of melt strength under the use condition above 200.degree. C., the presence of odor due to the crosslinking assistant, and in obtaining enhancement of physical properties other than melt strength such as heat-resisting property and crystallizing temperature.
In the case where a metallocene catalyst system is used, the processes proposed above may be effective to enhance melt tension to a certain degree under the measurement condition of polyolefin at 190.degree. C., but are not sufficient to ethance melt tension at higher temperature, for example, under use condition above 200.degree. C. so that enhancement of melt strength is continuously desired.
In recent years, examples of copolymers of a non-conjugated diene and propylene or the like is reported which is polymerized by, the aid of a metallocene catalyst, for example, in Japanese Laid-open Patent AppIn. Nos. Hei. 5-222251 and Hei. 5-222121. In Examples of these publications, however, there is nowhere given the case of using metallocene catalysts. It cannot but be anticipated therefore that propylene copolymers obtained by copolymerizing propylene at a practical polymerization temperature above 45.degree. C. by the aid of a metallocene catalyst are low in stereoregularity and not satisfactory in molecular weight in addition to insufficient melt strength so that the copolymers are unable to use practically.
In Japanese Laid-open Patent Appln. No. Hei. 7-138327, there is reported an example for copolymerization of propylene and an .alpha.,.omega.-diene. In this case, however, the polymerization temperature is lower than 45.degree. C. at which a practical polymerizatkin is carried out and the molecular weight of the resultant copolymer is extremely low. What is more, since the catalyst is not carried on a support, mass production of copolymers in a practical process may be difficult.
In Japanese Laid-open Patent Appln. No. Hei. 8-92317, too, there is reported an example of copolymerization for propylene and a polyene. However, the polymerization temperature is lower than 45.degree. C. which is a practical polymerization temperature. Further, stereoregularity of the resultant copolymer is at any rate high at a polymerization of 25.degree. C. shown in Examples but lower at a polymerization temperature above 45.degree. C. During insertion of the total propylene, a ratio of a mis-inserted unit based on 2,1-insertion of propylene monomer is greater than 1.0% so that it cannot be said that control of high stereoregularity is made. Moreover, the catalyst system is not carried on a support so that mass production of the copolymer is impossible in a practical process.
This publication nowhere gives any description or suggestion on heat-resisting property of a copolymer of propylene or the like olefin with an .alpha.,.omega.-diene copolymerized by the aid of a metllocene catalyst system.
Accordingly, it is an object of the present invention to provide propylene copolymers possessing a narrow molecular weight distribution, high melt strength, heat-resisting property, a low MFR and excellent in particulate properties wherein the percentage of mis-insertion in the copolymer is highly controlled as well as a process for producing the copolymers wherein high stereoregularity is maintained even at a practical polymerization temperature.